The present invention is in the area of Light Emitting Diode (LED) technologies and pertains particularly to an LED module and control circuitry and methods for replacing incandescent elements in conventional hand-held lights including flashlights.
As the technology of LEDs has progressed, leading to increased brightness, the practicality of using LEDs for general illumination and in particular for flashlights has improved. The development of white LEDs has lead to a profusion of commercially available flashlight products. White LEDs are also used as back lights for graphic display devices, and various types of battery operated illumination products. A problem in the design of these products stems from the relatively high operating voltage of the white LEDs, which require approximately 3.5-4.0 Volts, as compared to approximately 1.8-2.5 Volts for LEDs of other colors. Many traditional incandescent lamp flashlight designs use two battery cells each having a voltage of 1.5 Volts; the 3.0 Volts thus developed is incapable of lighting the white LED. LEDs generally yield a light output which is substantially proportional to the current through the LED; the LED current (xe2x80x9cforward currentxe2x80x9d) is a nonlinear exponential function of the LED voltage, (xe2x80x9cforward voltagexe2x80x9d), similar to that of other semiconductor diodes.
One general approach to this problem has been to use more than two cells as the power source. Many commercial LED flashlights use three 1.5 Volt cells connected to the white LED(S) directly or via a small resistor. In devices of this type, the LED current depends strongly on the battery voltage and varies widely as the battery discharges. The LED current also varies to significant degree from unit-to-unit due to normal production variation in the LED forward voltage. These variations are undesirable because they typically cause the LED current to start off very high with a fresh set of batteries, operating the LED(s) at a current in excess of their rating, degrading their brightness and shortening their useful life; later, when the batteries are partially discharged, the current decreases, leading to a long regime of operation in which the light output is relatively weak.
Some products overcome this effect by using a linear current regulator circuit to regulate the LED current as the battery voltage. In operation, the difference in voltage between that of the battery and that of the LED(s) is dropped across the linear regulator circuit; this causes a decrease in efficiency as compared to the direct connection described above.
Another drawback to these methods is that if multiple LEDs are used, they are typically connected in parallel. If multiple LEDs are connected in series, the voltage required is then multiplied by the number of series connected LEDs. Although a high voltage battery comprised of many cells (for example, 8 cells to give 12 Volts) could be used, the size, weight and cost of the battery are increased due to the extra material needed to make up the many individual cells. If multiple LEDs are connected in parallel, the current in each of the LEDs will depend on the relative forward voltages of the particular LEDs used. If the LED forward voltages are much different, one of the LEDs may take more than its share of current leading to the brightness degradation and lifetime shortening noted above. The LED currents will generally only be equal if the devices are well matched in forward voltage. The sorting and matching needed to assure good current sharing adds cost and complexity to the manufacture of products which use parallel LEDs. The addition of resistors in series with the individual paralleled LEDs can improve the current sharing, with some decrease in efficiency.
Another general method of overcoming the difficulties brought on by the high voltage needed to operate white LEDs is to use an electronic power conversion circuit to derive a voltage higher than that of the battery. There are many circuit topologies that can perform this conversion, and they are well known in electronic power supply technology. A few have been successfully applied to the design of LED lighting products.
The particular topology described here is the boost converter, which uses an inductor as an energy storage device. The circuitry of the boost converter may be made from discrete electronic components, or by the use of one of a number of specialized integrated circuits which are sold for this purpose. The use of the boost converter overcomes many of the problems outlined above. The output voltage of the converter can be controlled in various ways so that the LED current need not depend strongly on the battery voltage, and can be regulated and precisely tailored to achieve specific goals, as will be described. Since the battery voltage can be increased by an arbitrary factor, any number of LEDs can be connected in series, obviating the need to parallel LEDs. The power loss due to imperfect conversion efficiency of the boost converter makes this approach less efficient than direct battery connection of the LED(s), but compares favorably with linear current regulator methods. Circuit complexity is the only other drawback to the boost converter approach.
Note that many electronic boost converter circuits, especially those that use integrated circuits, are susceptible to damage from reverse polarity connection of the battery. Since reversal of the batteries is a common mistake, it is desirable to include some form of reverse battery protection. Ideally, this should have minimal effect on power conversion efficiency and be simple and low cost.
Low battery indicators are of course not new in electronics or in flashlights. However, LED flashlights benefit more from an indicator than flashlights based on incandescent lamps. The reason for this is that whereas incandescent lamps generally show the user that the batteries are becoming weak by a noticeable change in the color of the light, LEDs display little or no change in the color or character of the light produced, even over a wide range of light intensity. Since the human eye perceives light intensity over a very wide range (108:1), small changes (2:1) are difficult to discern, and even larger changes (10:1) may not be obvious. Thus, it is difficult to tell if the light output of an LED flashlight has decreased, as compared to the light given with fresh batteries. A battery indicator lets the user know that the batteries are getting weak, even though the light output is still useable.
In the above description of prior art the inventor has drawn upon a number of documents available in the art. These documents are provided in an Information Disclosure Statement filed with the present patent application.
In addition to the above background, there are at the time of the filing of the present patent application a relatively large number of flashlight products implementing incandescent bulbs as a light source commercially available, and a relatively smaller number of such flashlights that use LEDs as a light source. An example of the former of particular interest to the present inventor are models of miniature flashlights using incandescent bulbs, and marketed under the registered tradename of Mag Light(trademark) products. It is an object of the present invention to provide easily-usable LED modules for retrofitting existing incandescent flashlight products, in particular those made by Mag Instrument, Inc., and sold under the tradename of Mag Light(trademark). Such a retrofit module should be designed to maintain the original twist on-off switch function of the flashlight, and preserve or improve the original mechanical ruggedness. The retrofit changes should be easy to make, even by those unskilled and unfamiliar with the construction of the flashlight. There should be no damage or permanent changes to the flashlight, and the retrofit modification should be reversible at a later time if desired
What is needed is a circuitry design that controls the LEDs to maintain a steady and constant level of illumination over an extended period of time, followed by a low battery state, during which the light output is allowed to decrease while keeping the batteries at a constant voltage, and during which time an indication mechanism warning of low battery strength is provided. A further need in the art is for a replacement module incorporating white-light LEDs that can easily, inexpensively, and modularly replace incandescent bulbs in existing lighting products, such as those made by Mag Instrument, Inc., and referenced above.
In a preferred embodiment of the present invention an LED illumination module for a flashlight is provided, comprising a housing, electronic control circuitry, and one or more LEDs connected in series and to the circuitry. The module is characterized in that the it is shaped to modularly replace an incandescent-bulb in an existing flashlight.
In preferred embodiments the circuitry provides two distinct operating modes, a first mode stabilizing current through the LEDs as battery voltage wanes, and a second mode activated at a selected voltage to stabilize battery voltage and to trigger a low-battery alert. The alert mechanism can be any one of a colored LED, an audio element, or intensity modulation of one or more of the LEDs. In some embodiments the control circuitry also comprises reverse polarity protection circuitry to avoid damage to components due to reversed battery polarity. This protection circuitry may comprise a MOSFET configured to block the flow of current of reverse polarity, and to conduct current of correct polarity, said MOSFET being turned on by a voltage higher than that of the battery and provided by a DC/DC voltage boost converter.
In another aspect of the invention, in a LED illumination module having one or more LEDs, a control system for regulating power conversion and illumination intensity of the LED module is provided, comprising a voltage boost DC/DC converter, operating from a battery and powering said LEDs, a first feedback mechanism for regulating LED current by controlling said voltage boost DC/DC converter, a second feedback mechanism for regulating battery voltage by controlling said voltage boost DC/DC converter, and a mechanism for automatically transitioning between the first and the second feedback mechanisms in response to a predetermined battery voltage threshold.
In a preferred embodiment the module can be retrofitted to an existing hand-held light. Also in a preferred embodiment there may be reverse polarity protection circuitry to avoid damage to components due to reversed battery polarity. In some cases the reverse polarity protection circuitry comprises a MOSFET configured to block the flow of current of reverse polarity, and to conduct current of correct polarity, said MOSFET being turned on by a voltage higher than that of the battery and provided by a DC/DC voltage boost converter.
In some embodiments of the control system a low battery indicator is provided, enabled to alert the user when transition between the first and the second feedback control modes occurs, or is about to occur. In some cases alerting the user is accomplished via an LED indicator. In other cases alerting the user may be accomplished via an audio sound producing device, or through intensity modulation of one or more of the LEDs.
In yet another aspect of the invention a method for controlling a LED module having one or more LEDs s provided, comprising steps of (a) providing power to the one or more LEDs through a DC/DC converter from a battery pack providing an input voltage; (b) feeding back a signal representative of LED current to the converter to maintain a substantially constant LED current as the batteries discharge and the input voltage decreases; (c) beginning at a preset value for input voltage, switching to a mode of battery voltage control feedback to the converter to reduce current to the LEDs in a manner to maintain the battery voltage at a substantially constant value.
In a preferred embodiment of the method there is a step (d) for initiating a low-battery alert mechanism at or near the pre-set input voltage. The alert mechanism may be one or another of lighting a color LED, an audio signal, or intensity modulation of the one or more LEDs.
In yet another aspect of the invention a method for retrofitting a flashlight having a body cylinder and an incandescent bulb mounted with a reflector in an insulator receptacle translatable by rotating a head assembly relative to the body cylinder, said translation opening or closing a contact to the body cylinder as an on-off mechanism, the method comprising steps of (a) removing the head assembly; (b) replacing the reflector with an adapter ring; (c) removing the incandescent bulb; (d) removing the translatable insulator receptacle, and replacing it with a LED module of substantially the same geometry, but comprising one or more white-light LEDs; and (e) replacing the head assembly.
In a preferred embodiment, in step (d), the LED module further comprises electronic control circuitry for converting and managing the transfer of battery power to the one or more LEDs. In some cases the control circuitry operates in a dual mode, a first mode controlling LED current to be substantially constant while battery voltage deteriorates, and a second mode allowing LED current to decrease while maintaining battery voltage substantially constant, the second mode triggered at a preset voltage value.
In embodiments of the present invention described in enabling detail below, for the first time a replacement module is provided that allows convenient retrofitting of flashlights to operation with white LEDs.